1. Field of the Invention
The invention relates to the field of fiber optics. It concerns, especially, a fiber-optic sensor for alternating electric fields or voltages, comprising
(a) a piezoelectric sensor element; PA1 (b) an optical fiber having an entrance end and an exit end, which optical fiber is fixed at least partially to the sensor element so that a dimensional change of the sensor element in an electric field leads to a change in the length of the fiber; and PA1 (c) means for measuring the field-dependent change in length of the fiber; PA1 (d) the fiber is a bimodal fiber, the parameters of which are selected so that the LP.sub.01 fundamental mode and the even LP.sub.11 mode can propagate within it; PA1 (e) upstream of the entrance end of the bimodal fiber there is disposed a coherent light source which excites the two modes of the bimodal fiber; and PA1 (f) the means for measuring the field-dependent change in length of the fiber comprise optical means for the separation of the near-field and remote-field signals present at the exit end of the bimodal fiber, detectors for the conversion of the near-field and remote-field signals into corresponding electrical signals and electronic means for obtaining the length change information from these converted near-field and remote-field signals. PA1 (a) a first lens, disposed directly at the exit end of the bimodal fiber, for the collimation of the two modes emerging from the bimodal fiber to form a parallel beam; PA1 (b) a first beam splitter which is disposed downstream of the first lens and which splits up the parallel beam into two partial beams; PA1 (c) a second lens with a downstream first coupling-out fiber, which second lens focuses the first of the two partial beams so that the end surface of the bimodal fiber is imaged onto the entrance end of the first coupling-out fiber; and PA1 (d) a third lens with at least one downstream second coupling-out fiber, which third lens concentrates the second partial beam so that the entrance end of the second coupling-out fiber is still situated within the optical remote field of the bimodal fiber, but the light is already efficiently coupled into the second coupling-out fiber. PA1 (a) the bimodal fiber together with the sensor element and the optical means for the separation of the near-field and remote-field signals emerging at the exit end of the bimodal fiber forms a separate sensor head; PA1 (b) the light source, the detectors and the electronic means for obtaining the length change information from these near-field and remote-field signals are part of a separate sensor electronic system; PA1 (c) the light source is optically connected via a polarization-retaining monomode fiber with the entrance end of the bimodal fiber; and PA1 (d) the detectors are optically connected with the optical means by separate glass fibers in the form of multimode fibers.
Such a fiber-optic sensor is known, for example, from EP-Al-0,316,619.
2. Discussion of Background
Various publications such as, for example, European Patent Applications EP-Al-0,316,619 and EP-Al-0,316,635 or the articles by K. Bohnert and J. Nehring in Appl. Opt. 27, pp. 4814-4818 (1988), or Opt. Lett. 14, pp. 290-292 (1989), have already contained descriptions of fiber-optic sensors for measuring electric fields and voltages.
The measurement principle employed in this case is based on the inverse piezoelectric effect in materials with selected crystal symmetry. The temporally periodic dimensional change which is experienced by an appropriate piezoelectric body in an alternating electric field is transferred to a glass fiber fixed to the body. The change in the length of the fiber is then proportional to the amplitude of the field or voltage and is interferometrically measured and evaluated.
It is possible to use various types of glass fiber interferometers for the interferometric measurement. As a result of its simplicity, of these types the bimodal fiber interferometer disclosed in the article by B. Y. Kim et al., Opt. Lett. 12, pp. 729-731 (1987) is of particular interest. In the case of this interferometer, the parameters of the sensor fiber are selected so that precisely two modes (the LP.sub.01 fundamental mode and the even LP.sub.11 mode) can propagate in the fiber.
As shown in FIG. 1A with reference to the principle of a fiber-optic 1 field sensor, in the case of the bimodal fiber interferometer light is passed from a coherent light source 1, e.g. a laser diode, through a bimodal fiber 3, which is fixed to a piezoelectric sensor element 2 for the electric field E. At the fiber end it is then possible to observe an interference pattern which arises from the superposition of these two modes. A change in the length of the fiber leads to a differential phase shift between the two modes, which is expressed in a corresponding change in the interference pattern. FIG. 1B shows such interference patterns for three characteristic phase differences n2pi, (2n+1)(pi/2) and (2n+1)pi.
The two mutually adjacent substructures of the interference pattern (indicated in FIG. 1B by semiellipses) are detected by two detectors 5a and 5b (e.g. in the form of photodiodes). Two signals V.sub.11 and V.sub.12 which are phase-shifted by 180.degree. are present at the output thereof: EQU V.sub.11 =(1/2)V.sub.0 (1+acos.PHI.(t)) (1) EQU V.sub.12 =(1/2)V.sub.0 (1-acos.PHI.(t)) (2)
where .PHI.(t)=Asin.OMEGA.t+.THETA.(t). The phase shift .PHI.(t) between the two modes is thus composed of a temporally periodic component Asin.OMEGA.t which is generated by the alternating field to be measured (in this case, A is proportional to the amplitude of the field) and an arbitrary phase term .THETA.(t), which may likewise alter with time, for example, in consequence of temperature-dependent fluctuations of the fiber length. Finally, V.sub.0 is proportional to the optical power and a is a measure of the interference contrast.
The target term Asin.OMEGA.t is frequently obtained by a homodyne detection process (FIG. 1C) from the output signals of the detectors 5a and 5b (for a fiber-optical sensor with a monomode fiber, see in this connection: D. A. Jackson et al., Appl. Opt. 19, pp. 2926-2929 (1980); a corresponding fiber-optic sensor with a bimodal fiber is described in the older European Application No. 90123660.4). In this process, the sensor fiber (in the example shown in FIG. 1C, the bimodal fiber 3) is additionally passed via a piezoelectric modulator 4. By means of this modulator 4, the phase difference .PHI.(t) is set to +(pi/2) or -(pi/2) (modulo 2pi). To this end, the modulator 4 is a component of a control loop, which consists of the detectors 5a and 5b, a subtractor 7, a quadrature controller 8 and a feedback line 6 and which sets the difference voltage EQU V=V.sub.11 -V.sub.12 =V.sub.0 acos.PHI.(t) (3)
correspondingly to zero.
The two components Asin.OMEGA.t and .THETA.(t) of the phase shift are thus precisely balanced by the modulator via a corresponding (offset) change in length of the fiber. The voltage present at the modulator 4 then contains a slowly varying component, which is proportional to .THETA.(t) and a periodic component which is proportional to Asin.OMEGA.t. The target component Asin.OMEGA.t is filtered out by means of a high pass filter 9 and can be picked off at the signal output 10. As a result of this, the output signal is independent of any possible fluctuations of the laser intensity (i.e. V.sub.0) and of the interference contrast a.
In addition to the homodyne process, the literature describes certain further detection processes, which have the advantage that an additional modulator in the region of the interferometer can be dispensed with, but which processes require instead a more complicated sensor electronic system for the signal demodulation, which moreover frequently exhibits a low degree of precision. Examples of this are the synthetic heterodyne process (J. H. Cole et al., IEEE J. Quant. Electr. QE-18, pp. 694-697 (1982)), the homodyne process with a phase-modulated carrier signal (A. Dandridge et al., IEEE J. Quant. Electr. QE-18, pp. 1647-1653 (1982)), and processes in which two interferometer signals are generated by optical means, which signals are phase-shifted by 90.degree. relative to one another (S. K. Sheem et al., Appl. Opt. 21, pp. 689-693 (1982)).
In a series of practical applications of the sensor (e.g. in voltage measurement in outdoor substations) it is possible for relatively large spacings to occur between the actual sensor head and the sensor electronic system (10 m to a few 100 .mu.). It is inexpedient to bridge these spacings by the bimodal fiber itself, since the influence of external disturbing factors (temperature fluctuations, mechanical vibrations etc.) increases with increasing fiber length in a corresponding manner and the signal/noise ratio deteriorates. The light feed from the laser diode to the interferometer and the return of the output signals of the interferometer should rather take place via separate glass fibers which are not a component of the interferometer.
In the above described homodyne process, having an active phase modulator, it would however in addition to the connecting glass fibers also be necessary to provide an electrical connection (the feedback line 6) between the sensor electronic system and the sensor head to drive the modulator 4. The attractiveness of a sensor operating with this type of interferometer would thus be very limited.